In spite of claims by pharmaceutical companies that they do not discharge their products into the water supply, federal researchers have discovered that waters downstream of pharmaceutical plants are more heavily contaminated with drug residue than waters elsewhere in the country.

In one study, conducted by scientists at the U.S. Geological Survey (USGS), researchers tested the water entering two water treatment plants down the sewer line of several pharmaceutical factories, as well as at other plants not receiving sewage from drug plants. Researchers discovered drugs at “much higher detection frequencies and concentrations” at the plants receiving effluent from pharmaceutical factories. Drugs detected included opiates, a barbiturate and a tranquilizer.

In a second study, researchers from the Environmental Protection Agency tested the water entering a wastewater treatment plant in the city of Kalamazoo, Mich., down the sewage line from a Pfizer drug factory. They found that the water entering the plant was exceptionally high in levels of the antibiotic lincomycin, which the factory was producing at that time.

“There’s some product going down the drain,” said Bruce Merchant, the city’s public services director.

Prior studies have shown that lincomycin can cause genetic mutations, and that it encourages the growth of cancer cells when combined with minute concentrations of a number of other drugs that are common in surface water.

The two studies are among the first to test longstanding claims by the pharmaceutical industry that factory emissions are not a significant source of drug residue in drinking water supplies.

Research outside of the United States also suggests that pharmaceutical companies are major sources of drug pollution. In Switzerland, a test by drug company Roche found that a full 0.2 percent of active drug ingredients enter the environment during the manufacturing process. Another study found that 100 pounds of the antibiotic ciproflaxin were entering the water every day from a drug factory in India.